A copy of something that got completey twisted to become something else.
This phrase comes from the fact that lithium is used to help cure bi-polar disorders, thus the lithium copy is the other personality of an imaginative bi-polar person who consists of the original person/object and its lithium copy.
1. Jason is Paul's lithium copy; Jason enjoys throwing kittens off buildings, but Paul loves kittens and runs a nursery for them.
2. The deathcore band did a remix of the boyband's love song. The remix is a total lithium copy of the original, and is about a necropheliac fucking the dead body of his suicidal girlfriend.